Start by soaking the cashews in filtered water, covered, for 2 hours, or overnight.
Place all the tart pans on a cookie sheet.
In the food processor, add the chocolate sandwich cookies, and pulse a few times at first to break them up, and then turn the food processor on continuously to make a fine sand with the cookies.
Add the butter, and mix thoroughly. Divide among the 6 tart pans, then press the crust up against the rim of the tart pan first, forming a compact crust. Then, with whats left, press down into the bottom, ensuring even coverage. Make sure to pack tightly. Move these to the refrigerator while you prepare the other ingredients.
For the cashew cream, drain the cashews and place them in the blender, along with the oat milk, vanilla, and sugar. Turn the blender on, gradually increasing the speed to high, to make a very smooth, and creamy sauce. The consistency should be just slightly thinner than pudding. Place the mixture in your piping bag with the #3 tip, set on a plate and put in the refrigerator while we make the chocolate pudding.
Rinse out your blender container, and add the cubed avocado, cocoa powder, sugar, and oat milk. Blend until very creamy and smooth, about 45-60 seconds.
Remove tart pans from refrigerator, and divide the chocolate avocado mousse among the tarts, flattening the surface with an offset spatula or butter knife.
Remove the cashew cream from the refrigerator, and starting at the center of a tart, make spirals, keeping the lines about 1/4 - 1/2 inch apart. Using a toothpick, start at the center, and drag the toothpick towards the rim in six equal lines, working your way around the tart. In the middle of those lines, drag the toothpick in the opposite direction, from the rim to the center, to make a cool spiderweb pattern.
Refrigerate for two hours. When ready to serve, gently remove rim from tart pan, then use a spatula or butter knife to help separate the bottom of the tart pan from the tart itself. Plate, and serve!